Recent years have seen the development of biometric authentication technology for authenticating a user of an apparatus or system based on a biometric image representing the user's biometric information such as a palm or finger vein pattern or a fingerprint or palm print. A biometric authentication device using such biometric authentication technology acquires, as a biometric input image, a biometric image representing, for example, the biometric information of a user who wishes to use the biometric authentication device. Then, the biometric authentication device compares the user's biometric input information represented by the biometric input image with registered biometric information, i.e., the biometric information prestored in the form of a biometric image for each registered user. If it is determined as the result of the comparison that the biometric input information matches the registered biometric information, the biometric authentication device authenticates the user as being a registered user. The biometric authentication device then permits the authenticated user to use the apparatus in which the biometric authentication device is incorporated or some other apparatus connected to the biometric authentication device.
When such a biometric authentication device is used, the user has to wait until the biometric authentication process is completed, before he can use the desired apparatus. Therefore, if the time taken to complete the biometric authentication process is long, the user will be inconvenienced. Especially, in the case of a one-to-N matching scheme in which the biometric input information is compared against a set of a plurality of pieces of registered biometric information each prestored as a registered biometric image in the biometric authentication device, the biometric authentication device has to perform a plurality of comparisons. This inevitably increases the time taken to complete the biometric authentication process.
To address this problem, a technique has been developed that classifies biometric input information into one of a plurality of classes, based on a feature quantity extracted from the biometric input information, and that compares the biometric input information only with the registered biometric information that belongs to the same class as the biometric input information (for example, refer to Japanese Laid-open Patent Publication No. 2002-133416 and Japanese Laid-open Patent Publication No. 2006-39777). By using this technique, the number of comparisons to be performed can be reduced, because the number of pieces of registered biometric information used for comparison decreases. As a result, even when the one-to-N matching scheme is employed, the biometric authentication device can reduce the overall time taken to complete the biometric authentication process. In the known art, when the biometric input information is a fingerprint, a feature, such as ridge spacing, center position of fingerprint, or ridge direction, is used to classify the biometric input information.
In the user of the biometric authentication technology, it is desirable that the characteristic features of the biometric information be captured clearly in the biometric image. However, if the user's body part as the biometric information to be used for matching is not properly positioned relative to the sensor for capturing the biometric information, a portion of the biometric information to be used for matching may not be captured in the biometric image or the biometric information captured may not be clear enough. In such cases, the biometric authentication device may not be able to extract from the biometric input information the feature quantity to be used for classifying the biometric input information, and may thus become unable to accurately classify the biometric input information. If the biometric input information is erroneously classified as belonging to a class different from the class to which it should belong, the biometric authentication device is unable to compare the biometric input information with the registered biometric information of the corresponding user. This results in a degradation of the authentication accuracy.
In view of the above, a technique has been developed that notifies the user to have his biometric information reread by the sensor to re-acquire the biometric image if it is determined that the biometric input information has not been captured correctly (for example, refer to Japanese Laid-open Patent Publication No. 2001-167268 and Japanese Laid-open Patent Publication No. 2003-256815). According to this technique, it is determined whether the image has been captured correctly or not, based on the density distribution of the image representing the biometric input information, or based on whether the desired feature point of the biometric information has been successfully extracted from the image or on the position of the feature point.